Well known in the art are chemical processes for coloring stainless steel using a coloring aqueous solution of so-called chromic acid-sulfuric acid series, including, for example, chromic anhydride-sulfuric acid series, sodium bichromate-sulfuric acid series and potassium bichromate-sulfuric acid series. Preferred examples of such coloring solutions are disclosed in UK Patent specifications Nos. 1,122,172 and 1,122,173. When stainless steel is immersed in such a coloring solution colors of various shades may develop on the surfaces of the stainless steel depending upon the time of immersion. This is because, although related to the composition of the steel, depending upon the extent and state of oxide films formed on the surfaces of the steel by particular redox reactions the color shade thereof varies. This variation of the color shade appears continuously. Accordingly, it is possible to develop a color of the predetermined shade by controlling the time of immersion (the processing time). A convenient method for controlling the color shade is disclosed in Japanese Patent Post-examination Publication No. 52-25,817. The principle underlying the disclosed method is such that the difference between the potential of the steel, which is immersed in the coloring solution and on which oxide films are being formed, and the potential of a reference electrode disposed in the coloring solution varies in accordance with the extent of the oxide films being formed on the steel, i.e. in accordance with the color shade, and therefore the color shade may be controlled by measuring and using the above-mentioned potential difference as a measure. Thus, if an article of stainless steel immersed in a coloring solution is taken out of the solution when the above-mentioned potential difference has reached a predetermined value, a colored article having a shade which corresponds to the potential difference value may be obtained. This method in which the potential difference is utilized as a measure for controlling the color shade is very useful when the coloring process is carried out batchwise. In fact when a stainless steel article to be colored is immersed batchwise in a coloring solution, coloring proceeds substantially uniformly over the whole surfaces of the article, and in consequence, if the immersed article is taken out of the coloring solution at the time the potential difference has reached the value corresponding to the desired color shade, it is possible to achieve the substantially same color shade over the whole surfaces of the article.
If uniform coloring can be realized by a continuous procedure, the productivity of the coloring process will be greatly increased. It would be very advantageous if it is possible to develop a certain predetermined color over the whole surfaces of a stainless steel strip, for example, by causing the strip to continuously pass through a coloring solution. More particularly, it is desired to color a stainless steel strip uniformly over the whole length of the strip by providing a vessel containing a coloring solution between a pay-off roll and a wind-up roll and continuously moving the stainless steel strip so that the strip unwound from the pay-off roll may be caused to pass through the vessel and wound up by the wind-up roll while forming colored coatings thereon during its passage through the coloring solution in the vessel.
However, with such a continuous procedure the state of the colored coating being formed on the strip passing through the coloring solution varies continuously from the initial state immediately after the strip is immersed in the coloring solution, at which state substantially no colored coating is formed, to the final state immediately before the strip is taken out of the coloring solution, at which state the colored coating has grown to a certain thickness. Accordingly, even if a potential difference between the moving strip at a selected position and a reference electrode located in the coloring solution is measured, the measured value will be affected by the varying states of the colored coating which extends in front and rear of the selected position, and will not precisely reflect the state of the colored coating at the selected position. In other words, it is unreasonable to utilize the above-mentioned measured potential difference as such as a detected signal for controlling the state of the formed colored coating. By way of an example, let us suppose that a suitable contact means for detecting a potential is fixedly mounted so that the steel of the strip may come in contact with said means immediately before the strip leaves the coloring solution, and the potential difference is measured between the steel in contact with said means and a reference electrode located in the coloring solution. The measured value however does not solely correspond to the colored coating formed on that portion of the strip which is going to leave the coloring solution (the colored coating at the detecting position), rather the value is influenced by the colored coating formed on broad areas of the strip which extends from the detecting position into the coloring solution (the nature of the colored coating gradually varies depending upon the distance from the detecting position).
In addition, with the continuous procedure, the composition of the coloring solution and the concentrations of ingredients thereof inevitably vary with time as the process is continued. As the process proceeds the concentration of Fe.sup.3+ resulting from the redox reaction involved as well as the proportions of Cr.sup.3+ and Cr.sup.6+ in the coloring solution vary. Furthermore, loss of water due to evaporation occurs, since the process is normally carried out at an elevated temperature. It is practically impossible to maintain the composition and concentrations of the coloring solution always constant during the continuous procedure. Fresh surfaces of a stainless steel strip are successively introduced into the coloring solution which is gradually varying with time, and thus the colored coating formed on the strip is also influenced by the variation of the coloring solution.
The problems discussed above are peculiar to the continuous procedure. The batch procedure is not accompanied by such problems. In the batch procedure the colored coating of substantially the same thickness grow similarly anywhere of the surfaces of an article to be colored, and all portions of the surfaces of the article contact with the coloring solution of the same composition and concentrations at anytime irrespective of the variation with time of the composition and concentrations of the coloring solution.
Japanese Patent Post-examination Publication No. 58-25747 discloses a proposal for solving the above-mentioned problems involved in the continuous procedure. According to this publication, as a surface to be colored is moved, the coloring solution is also moved at the same velocity. However, such a proposal requires an apparatus of a larger scale, and does not yet ensure that the composition and concentrations of the coloring solution may be always maintained constant as the process proceeds. Japanese Patent Post-examination Publication No. 56-4151 discloses a method in which the determination of the potential difference used in the batch process is as such applied to the continuous process, that is a method in which a potential difference between a continuously running stainless steel strip and a reference electrode disposed in a coloring solution is measured. More particularly, a potential difference between the steel of the strip passing over a certain point and the reference electrode is measured and is utilized for controlling the formation of colored coating. However, color control can not be exactly carried out by such a method for the reasons discussed above and as illustrated hereinafter in control examples. Japanese Patent Laid-open Publication No. 58-167778 proposes a method in which the color phase instead of any potential difference is utilized as a measure for controlling a continuous process of coloring a stainless steel strip. However, the determination of the continuously varying color phase poses a problem in the precision of the measurement.